make sure the plugin can detect if a user changes setting "Always use LDAP for auth" while users exist

This can result in strange, hard to diagnose errors for users who don't understand how the plugin works. Detecting this and informing the user of possible bugs would be helpful, I think.

No files, tests, or entry points are named. Start by locating the handling for the "Always use LDAP for auth" setting and the code that manages existing users, then determine where a change can be detected and reported. Done means users are informed about the possible authentication problems when the setting changes while users exist.

Written by the indexing model from the issue text.